The United States is urging tightened sanctions against Iran following exposure of an alleged assassination plot that Secretary of State Hillary Clinton said shows a "dangerous escalation" of Iranian involvement in terrorism. The Tehran government continues to vigorously deny that it plotted to kill the Saudi ambassador to Washington.

The Obama administration has launched a wide-ranging diplomatic effort to fulfill its pledge to hold Iran accountable for its alleged role in a plot to kill Saudi Ambassador to Washington Adel al-Jubeir.

A day after the Justice Department announced criminal charges in the case, U.S. diplomats reached out to governments around the world to increase pressure on Iran, including tighter enforcement of U.N. sanctions already in place against Tehran.

U.S. officials say an Iranian-American in Texas, working with an official of the elite Quds Force of the Iranian Revolutionary Guards Corps, sought to engage a Mexican drug cartel to stage a bomb attack in a Washington restaurant to kill the influential Saudi envoy.

The Iranian-American, Manssor Arbabsiar, was dealing with a U.S. informant posing as a Mexican drug gang member, and was arrested last month - averting what officials here say would have been a "massive act of terror."

Secretary of State Hillary Rodham Clinton greets audience member after speaking on American global leadership, Wednesday, Oct. 12, 2011, in Washington.

In a Washington speech Wednesday, Secretary of State Hillary Clinton said the alleged conspiracy, directed by elements of the Iranian government, was an act of recklessness.

"This plot, very fortunately disrupted by the excellent work of our law enforcement sand intelligence professionals, was a flagrant violation of international and US law, and a dangerous escalation of the Iranian government's long-standing use of political violence and sponsorship of terrorism," said Clinton.

Clinton urged other countries to join in condemning what she termed "this threat to international peace and security" and said the United States will work with partners to increase pressure on, and the isolation of the Tehran government.

The State Department called in Washington-based diplomats to brief them on details of the case, while U.S. Ambassador to the United Nations Susan Rice had similar meetings with envoys of U.N. Security Council member states.

State Department Spokeswoman Victoria Nuland said U.S. officials have not decided whether to seek new international sanctions on Tehran, and that existing ones are having an impact.

"Its economy has not been growing," said Nuland. "We see measures of increasing desperation to try to circumvent sanctions and measures like this. But again, as I've said, we believe all countries should look hard at how they can tighten sanctions, how they can enforce sanctions, and whether sanctions are well enforced to the limits of their own national laws."

Within hours after disclosure of the alleged conspiracy, the Treasury Department announced targeted sanctions against five figures in the case including four officials of the Quds force.

Treasury Wednesday also penalized an Iranian commercial airline, Mahan Air, it said supports the Quds Force and the Lebanese militant group Hezbollah.

The White House meanwhile said the plotting clearly involved senior levels of the Quds Force, but spokesman Jay Carney declined to say if U.S. officials believe knowledge extended higher into the Iranian government.

Iran continued to call the plotting allegations baseless with the Tehran government filing a protest with the Swiss government, which represents U.S. interests in Iran.

Secretary Clinton discussed the issue here with the Swiss Ambassador to Tehran Livia Leu Agosti. In the meeting, scheduled before the alleged plot was made public, Clinton thanked the Swiss envoy for her role in gaining the release of three American hikers detained by Iran.